Seeding-free aqueous synthesis of zeolitic imidazolate framework-8 membranes: How to trigger preferential heterogeneous nucleation and membrane growth in aqueous rapid reaction solution

Abstract

Zeolitic imidazolate frameworks (ZIFs) have received attention for membrane separation applications due to their zeolite-like permanent porosity and tunable uniformly-sized micropores. Although aqueous room temperature synthesis has apparently opened up environmental friendly and efficient ways to synthesize ZIFs, it poses challenges for membrane preparation including unavoidable homogeneous nucleation. Many ZIF membranes prepared in an aqueous system are based on conventional secondary seeded growth techniques for zeolite membranes in spite of well-recognizing that the coordination chemistry of ZIFs is fundamentally different from the covalent chemistry of zeolites. In this study, we first applied a support-surface activation approach to promote heterogeneous nucleation followed by crystal growth in the aqueous system. Continuous well-intergrown ZIF-8 membranes were successfully grown on α-alumina porous support using zinc acetate and showed relatively-high hydrogen permeance of 6.9×10−7mol/m2sPa with corresponding ideal separation selectivity of 13.6 for the hydrogen/methane. The competitive interaction between the coordination of constructing framework and zinc–acetate interaction by carboxylate functionality of acetate anions is essential to control heterogeneous nucleation and membrane growth. Avoiding the seeding process and reducing the use of organic solvents can provide potential for improving a reproducible, scalable, and commercializable process configuration for membrane preparation.